[media] s5p-fimc: Fix error in the capture subdev deinitialization

Make sure the subdev pointer is cleared when the subdev object
has been freed.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>