[media] media: usb: uvc: use vb2_ops_wait_prepare/finish helper

This patch drops driver specific wait_prepare() and wait_finish()
callbacks from vb2_ops and instead uses the the helpers
vb2_ops_wait_prepare/finish() provided by the vb2 core, the lock member
of the queue needs to be initalized to a mutex so that vb2 helpers
vb2_ops_wait_prepare/finish() can make use of it.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>