spi: spi-ath79: Use clk_prepare_enable and clk_disable_unprepare

Clocks should be prepared and unprepared, fix this by using
clk_prepare_enable() and clk_disable_unprepare() instead of
clk_enable() and clk_disable().

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>