mmc: pxamci: prepare and unprepare the clocks

Add the clock prepare and unprepare call to the driver set_ios calls
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: Ulf Hansson <ulf.hansson@linaro.org>