machine: move SPI Tx function into separate file

This makes the code a bit cleaner because ErrTxInvalidSliceSize isn't
redefined in every file that uses SPI and Mode0/Mode1/Mode2/Mode3 is
defined for every target that uses SPI.

src/machine/machine_rp2040_spi.go

@@ -38,10 +38,9 @@ type SPIConfig struct {
 }
 
 var (
-	ErrLSBNotSupported    = errors.New("SPI LSB unsupported on PL022")
-	ErrTxInvalidSliceSize = errors.New("SPI write and read slices must be same size")
-	ErrSPITimeout         = errors.New("SPI timeout")
-	ErrSPIBaud            = errors.New("SPI baud too low or above 66.5Mhz")
+	ErrLSBNotSupported = errors.New("SPI LSB unsupported on PL022")
+	ErrSPITimeout      = errors.New("SPI timeout")
+	ErrSPIBaud         = errors.New("SPI baud too low or above 66.5Mhz")
 )
 
 type SPI struct {