cpu/cc2538: spi: unify spi_transfer_bytes()
cpu/cc2538: spi: unify spi_transfer_bytes()

Use a common helper function to read/write the data register.

OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,miri64/RIOT,basilfx/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT,OlegHahm/RIOT,jasonatran/RIOT,basilfx/RIOT,jasonatran/RIOT,yogo1212/RIOT,jasonatran/RIOT,basilfx/RIOT,kYc0o/RIOT,kYc0o/RIOT,yogo1212/RIOT,OTAkeys/RIOT