spi: ath79: Simplify ath79_spi_chipselect()

First of all this callback was slightly misused to setup the clock
polarity at the beginning of a transfer. Beside being at the wrong
place, it is also useless as only SPI mode 1 is supported. Instead
just make sure the base value used for IOC is suitable to start a
transfer by clearing the clock and data bits during the controller
setup.

This also remove the last direct usage of the GPIO API, so we can
remove the direct dependency on GPIOLIB.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>