omap3isp: Set cam_mclk rate directly

Now that the cam_mclk rate changes are back-propagated to dpll4_m5_ck we
can set the cam_mclk rate directly instead of manually setting the rate
of the parent clock.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Tested-by: Sakari Ailus <sakari.ailus@iki.fi>