V4L/DVB (6944a): Fix Regression VIDIOCGMBUF ioctl hangs on bttv driver

Fix bttv VIDIOCGMBUF locking like done in commit
820eacd8. 

Signed-off-by: Gregor Jasny <gjasny@web.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>