[media] uvcvideo: Change result code of debugfs_init to void

The device driver should keep going even if debugfs initialization fails.
So, change the return type to void.

Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>