[media] adv7180: Fix remove order

The mutex is used in the subdev callbacks, so unregister the subdev before the
mutex is destroyed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>