media: v4l: Support passing media pad argument to v4l2_get_link_freq()

v4l2_get_link_freq() accepts a V4L2 control handler for now, but it needs
to take struct media_pad argument in order to obtain the link frequency
using get_mbus_config() pad op. Prepare for this by allowing struct
media_pad as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>