ASoC: pxa2xx-ac97: prepare and unprepare the clocks

Add the clock prepare and unprepare call to the driver initialization
phase. This will remove a warning once the PXA architecture is migrated
to the clock infrastructure.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mark Brown <broonie@linaro.org>