S3C64XX: Fix get_rate() for ARMCLK

If the requested clock is faster than the parent clock then the
parent clock is the closest we can get to the request so we need
to return that instead of the requested clock.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>