[media] v4l: subdev: tolerate null in media_entity_to_v4l2_subdev

Return NULL, if a null entity is parsed for it's v4l2_subdev

Currently, the callers of media_entity_to_v4l2_subdev() need to
make sure that the subdev is non-NULL, for example, when it is
obtained from media_entity_remote_pad().

As this is a recurring pattern, add the check at the macro in order
to avoid additional checks at the callers.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>