[media] em28xx-video: Swap release order to avoid lock nesting

vb2_fop_release might take the video queue mutex lock.
In order to avoid nesting mutexes the private mutex is taken after the
fop_release has finished.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>