v4l: xilinx: tpg: Add a goto label for error handling

Calling media_entity_cleanup() when subdev == NULL results in
kernel panic. Add a goto label for error handling with
subdev == NULL.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>