cpu/cc2538: fix spi_transfer_bytes()
cpu/cc2538: fix spi_transfer_bytes()

Always wait for RNE before reading DR.
Fixes always reading in the !out_buf case.

authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,x3ro/RIOT,toonst/RIOT,jasonatran/RIOT,kYc0o/RIOT,josephnoir/RIOT,ant9000/RIOT,josephnoir/RIOT,OTAkeys/RIOT,mtausig/RIOT,x3ro/RIOT,ant9000/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,josephnoir/RIOT,x3ro/RIOT,jasonatran/RIOT,ant9000/RIOT,x3ro/RIOT,kaspar030/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,basilfx/RIOT,aeneby/RIOT,OlegHahm/RIOT,smlng/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,smlng/RIOT,mtausig/RIOT,authmillenon/RIOT,mtausig/RIOT,aeneby/RIOT,mtausig/RIOT,OTAkeys/RIOT,smlng/RIOT,smlng/RIOT,OlegHahm/RIOT,kYc0o/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,aeneby/RIOT,kaspar030/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,yogo1212/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT,basilfx/RIOT,RIOT-OS/RIOT,x3ro/RIOT,basilfx/RIOT,authmillenon/RIOT,aeneby/RIOT,toonst/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,miri64/RIOT,yogo1212/RIOT,ant9000/RIOT,toonst/RIOT,toonst/RIOT,josephnoir/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,mtausig/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,smlng/RIOT,yogo1212/RIOT,aeneby/RIOT,RIOT-OS/RIOT,yogo1212/RIOT