Move the lock logic to it's own function. There is less code duplication
and cocci is much happier.

Fix the following cocci warning:
drivers/media/dvb-frontends/stv090x.c:799:1-7: preceding lock on line 768
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>