[media] adv7604: set is_private only after successfully creating all controls

is_private was set right after creating each control, but the control pointer
might be NULL in case of an error. Set it after all controls were successfully
created, since that guarantees that all control pointers are non-NULL.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>