media: usbtv: Implement wait_prepare and wait_finish

media: usbtv: Implement wait_prepare and wait_finish

This driver is currently specifying a vb2_queue lock,
which means it straightforward to implement wait_prepare
and wait_finish.

Having these callbacks releases the queue lock while blocking,
which improves latency by allowing for example streamoff
or qbuf operations while waiting in dqbuf.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>