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>

src/machine/machine_k210.go

@@ -419,7 +419,7 @@ type SPIConfig struct {
 // Configure is intended to setup the SPI interface.
 // Only SPI controller 0 and 1 can be used because SPI2 is a special
 // peripheral-mode controller and SPI3 is used for flashing.
-func (spi SPI) Configure(config SPIConfig) error {
+func (spi *SPI) Configure(config SPIConfig) error {
 	// Use default pins if not set.
 	if config.SCK == 0 && config.SDO == 0 && config.SDI == 0 {
 		config.SCK = SPI0_SCK_PIN
@@ -476,7 +476,7 @@ func (spi SPI) Configure(config SPIConfig) error {
 }
 
 // Transfer writes/reads a single byte using the SPI interface.
-func (spi SPI) Transfer(w byte) (byte, error) {
+func (spi *SPI) Transfer(w byte) (byte, error) {
 	spi.Bus.SSIENR.Set(0)
 
 	// Set transfer-receive mode.