media: omap3isp: Call video_unregister_device() unconditionally

video_unregister_device() can be called on a never or an already
unregistered device. Drop the redundant check.

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+samsung@kernel.org>