V4L/DVB (13372): staging/go7007: fix mutex function usage for s2250

Fix mutex function usage, which was overlooked in a previous patch.

Signed-off-by: Pete Eberlein <pete@sensoray.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>