media: Make parameter of media_entity_remote_pad() const

The local pad parameter in media_entity_remote_pad() is not modified.
Make that explicit by adding a const modifier.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>