spi/ath79: remove superfluous chip select code

The spi_bitbang driver calls the chipselect function
of the driver from spi_bitbang_setup in order to
deselect the given SPI chip, so we don't have to
initialize the CS line here.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>