spi: ath79: always call chipselect function

spi-bitbang has to call the chipselect function on the ath79 SPI driver
in order to communicate with the SPI slave device, as the ath79 SPI
driver has three dedicated chipselect lines but can also be used with
GPIOs for the CS lines.

Fixes commit 4a07b8bc

 ("spi: bitbang: Make chipselect callback optional")

Signed-off-by: David Bauer <mail@david-bauer.net>
Link: https://lore.kernel.org/r/20210303160837.165771-1-mail@david-bauer.net
Signed-off-by: Mark Brown <broonie@kernel.org>