refactor: use *SPI everywhere to make consistant for implementations.

Fixes #4663 "in reverse" by making SPI a pointer everywhere, as discussed
in the comments.

Signed-off-by: deadprogram <ron@hybridgroup.com>

Author: deadprogram

src/machine/board_feather-stm32f405.go

@@ -186,15 +186,15 @@ const (
 )
 
 var (
-	SPI1 = SPI{
+	SPI1 = &SPI{
 		Bus:             stm32.SPI2,
 		AltFuncSelector: AF5_SPI1_SPI2,
 	}
-	SPI2 = SPI{
+	SPI2 = &SPI{
 		Bus:             stm32.SPI3,
 		AltFuncSelector: AF6_SPI3,
 	}
-	SPI3 = SPI{
+	SPI3 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: AF5_SPI1_SPI2,
 	}

src/machine/board_lgt92.go

@@ -84,10 +84,10 @@ var (
 	I2C0 = I2C1
 
 	// SPI
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus: stm32.SPI1,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 func init() {

src/machine/board_maixbit_baremetal.go

@@ -6,10 +6,10 @@ import "device/kendryte"
 
 // SPI on the MAix Bit.
 var (
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus: kendryte.SPI0,
 	}
-	SPI1 = SPI{
+	SPI1 = &SPI{
 		Bus: kendryte.SPI1,
 	}
 )

src/machine/board_mksnanov3.go

@@ -89,11 +89,11 @@ const (
 
 // Since the first interface is named SPI1, both SPI0 and SPI1 refer to SPI1.
 var (
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: AF5_SPI1_SPI2,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 const (

src/machine/board_nucleol031k6.go

@@ -86,11 +86,11 @@ var (
 	I2C0 = I2C1
 
 	// SPI
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: 0,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 func init() {

src/machine/board_stm32f469disco.go

@@ -59,11 +59,11 @@ const (
 // Since the first interface is named SPI1, both SPI0 and SPI1 refer to SPI1.
 // TODO: implement SPI2 and SPI3.
 var (
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: AF5_SPI1_SPI2,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 const (

src/machine/board_stm32f4disco.go

@@ -86,11 +86,11 @@ const (
 // Since the first interface is named SPI1, both SPI0 and SPI1 refer to SPI1.
 // TODO: implement SPI2 and SPI3.
 var (
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus:             stm32.SPI1,
 		AltFuncSelector: AF5_SPI1_SPI2,
 	}
-	SPI1 = &SPI0
+	SPI1 = SPI0
 )
 
 const (

src/machine/machine_atmega.go

@@ -257,7 +257,7 @@ type SPI struct {
 }
 
 // Configure is intended to setup the SPI interface.
-func (s SPI) Configure(config SPIConfig) error {
+func (s *SPI) Configure(config SPIConfig) error {
 
 	// This is only here to help catch a bug with the configuration
 	// where a machine missed a value.
@@ -330,7 +330,7 @@ func (s SPI) Configure(config SPIConfig) error {
 }
 
 // Transfer writes the byte into the register and returns the read content
-func (s SPI) Transfer(b byte) (byte, error) {
+func (s *SPI) Transfer(b byte) (byte, error) {
 	s.spdr.Set(uint8(b))
 
 	for !s.spsr.HasBits(s.spsrSPIF) {

src/machine/machine_atmega1280.go

@@ -927,7 +927,7 @@ func (pwm PWM) Set(channel uint8, value uint32) {
 }
 
 // SPI configuration
-var SPI0 = SPI{
+var SPI0 = &SPI{
 	spcr: avr.SPCR,
 	spdr: avr.SPDR,
 	spsr: avr.SPSR,

src/machine/machine_atmega1284p.go

@@ -71,7 +71,7 @@ func (p Pin) getPortMask() (*volatile.Register8, uint8) {
 }
 
 // SPI configuration
-var SPI0 = SPI{
+var SPI0 = &SPI{
 	spcr: avr.SPCR,
 	spsr: avr.SPSR,
 	spdr: avr.SPDR,