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_fe310.go

@@ -140,7 +140,7 @@ type SPIConfig struct {
 }
 
 // Configure is intended to setup the SPI interface.
-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
@@ -197,7 +197,7 @@ 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) {
 	// wait for tx ready
 	for spi.Bus.TXDATA.HasBits(sifive.QSPI_TXDATA_FULL) {
 	}