V4L: ivtv: Add locking to ensure stream setup is atomic

V4L: ivtv: Add locking to ensure stream setup is atomic

Starting an MPEG and VBI capture simultaneously caused errors in
the VBI setup: this setup was done twice when it should be done
only for the first stream that is opened.
Added a mutex to prevent this from happening.

(cherry picked from commit f885969196da6ae905162c0d1c5f0553de12cb40)

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>