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_nrf.go

@@ -5,15 +5,10 @@ package machine
 
 import (
 	"device/nrf"
-	"errors"
 	"runtime/interrupt"
 	"unsafe"
 )
 
-var (
-	ErrTxInvalidSliceSize = errors.New("SPI write and read slices must be same size")
-)
-
 const deviceName = nrf.Device
 
 const (