[media] xilinx: Use a local media device pointer instead

[media] xilinx: Use a local media device pointer instead

The function has a local variable that points to the media device; use
that instead of finding the media device under the entity.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>