V4L/DVB (9875): gspca - main: Fix vidioc_s_jpegcomp locking.

Signed-off-by: Jim Paris <jim@jtan.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>