spi/ath79: avoid multiple initialization of the SPI controller

Currently we are initializing the SPI controller in
the chip select line function, and that function is
called once for each SPI device on the bus. If a
board has multiple SPI devices, the controller will
be initialized multiple times.

Introduce ath79_spi_{en,dis}able helper functions,
and call those from probe/response in order to avoid
the mutliple initialization of the controller.

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