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_atsamd21.go

@@ -10,7 +10,6 @@ package machine
 import (
 	"device/arm"
 	"device/sam"
-	"errors"
 	"runtime/interrupt"
 	"unsafe"
 )
@@ -1274,10 +1273,6 @@ func (spi SPI) Transfer(w byte) (byte, error) {
 	return byte(spi.Bus.DATA.Get()), nil
 }
 
-var (
-	ErrTxInvalidSliceSize = errors.New("SPI write and read slices must be same size")
-)
-
 // Tx handles read/write operation for SPI interface. Since SPI is a syncronous write/read
 // interface, there must always be the same number of bytes written as bytes read.
 // The Tx method knows about this, and offers a few different ways of calling it.