spi/ath79: add missing HIGH->LOW SCK transition

The 'ath79_spi_txrx_mode0' function does not
set the SCK signal to LOW at the end of a word
transfer. This causes communications errors with
certain devices (e.g. the PCF2123 RTC chip).

The patch ensures that the SCK signal will be LOW.

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