[media] arv: fix sleep_on race

interruptible_sleep_on is racy and going away. In the arv driver that
race has probably never caused problems since it would require a whole
video frame to be captured before the read function has a chance to
go to sleep, but using wait_event_interruptible lets us kill off the
old interface. In order to do this, we have to slightly adapt the
meaning of the ar->start_capture field to distinguish between not having
started a frame and having completed it.

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