cpu/esp32: fix CS handling in spi_transfer_bytes
cpu/esp32: fix CS handling in spi_transfer_bytes

If `SPI_CS_UNDEF` is given as the `cs` parameter, CS pin must not be handled by the driver. Furthermore, if `cont` parameter is true, CS pin must not be disabled at the end of one transfer.
jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,josephnoir/RIOT,OTAkeys/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,mtausig/RIOT,kaspar030/RIOT,authmillenon/RIOT,basilfx/RIOT,x3ro/RIOT,mtausig/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,yogo1212/RIOT,miri64/RIOT,yogo1212/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,smlng/RIOT,OTAkeys/RIOT,josephnoir/RIOT,toonst/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,josephnoir/RIOT,ant9000/RIOT,x3ro/RIOT,toonst/RIOT,mtausig/RIOT,josephnoir/RIOT,x3ro/RIOT,OTAkeys/RIOT,kYc0o/RIOT,ant9000/RIOT,yogo1212/RIOT,OlegHahm/RIOT,josephnoir/RIOT,authmillenon/RIOT,toonst/RIOT,x3ro/RIOT,kaspar030/RIOT,smlng/RIOT,basilfx/RIOT,smlng/RIOT,kaspar030/RIOT,mtausig/RIOT,aeneby/RIOT,ant9000/RIOT,jasonatran/RIOT,mtausig/RIOT,kaspar030/RIOT,aeneby/RIOT,toonst/RIOT,smlng/RIOT,aeneby/RIOT,basilfx/RIOT,ant9000/RIOT,miri64/RIOT,yogo1212/RIOT,x3ro/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,basilfx/RIOT,kYc0o/RIOT,aeneby/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,smlng/RIOT,miri64/RIOT,toonst/RIOT,aeneby/RIOT