media: mc: Make media_entity_get_fwnode_pad() fwnode argument const

fwnode_graph_parse_endpoint() fwnode argument is now const, therefore make
media_entity_get_fwnode_pad() fwnode argument const as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>