[ARM] S3C64XX: Fix s3c64xx_setrate_clksrc

Some of the rate selection logic in s3c64xx_setrate_clksrc uses what
appears to be parent clock selection logic. This patch corrects it.

I also added a check for overly large dividers to prevent them from
changing unrelated clocks.

Signed-off-by: Werner Almesberger <werner@openmoko.org>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>