media: omap3isp: Unregister media device as first

While there are issues related to object lifetime management, unregister the
media device first when the driver is being unbound. This is slightly
safer.

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>