[ARM] S3C24XX: pwm-clock set_parent mask fix

Fix s3c24xx pwm-clock supports use of the wrong clock when calculating
the bitmask when configuring the parent clock from the set_parent call.

Signed-off-by: Dallas Foley <dfoley@techsol.ca>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>