spi-bcm2708: Prepare for Common Clock Framework migration

As part of migrating to use the Common Clock Framework, replace clk_enable()
with clk_prepare_enable() and clk_disable() with clk_disable_unprepare().
This does not affect behaviour under the current clock implementation.

Also add a missing clk_disable_unprepare() in the probe error path.

Signed-off-by: Noralf Tronnes <notro@tronnes.org>