media: v4l2-common: Constify media_pad argument to v4l2_get_link_freq()

media: v4l2-common: Constify media_pad argument to v4l2_get_link_freq()

The v4l2_get_link_freq() macro doesn't modify the pad argument. Make it
possible to call it with a const media_pad pointer.

Link: https://lore.kernel.org/r/20250822002734.23516-2-laurent.pinchart@ideasonboard.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>