spi: spi-ath79: fix initial GPIO CS line setup

The 'ath79_spi_setup_cs' function initializes the chip
select line of a given SPI device in order to make sure
that the device is inactive.

If the SPI_CS_HIGH bit is set for a given device, it
means that the CS line of that device is active HIGH
so it must be set to LOW initially. In case of GPIO
CS lines, the 'ath79_spi_setup_cs' function does the
opposite of that due to the wrong GPIO flags.

Fix the code to use the correct GPIO flags.

Reported-by: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org