V4L/DVB (13955): cx25821: fix double unlock in medusa_video_init()

medusa_set_videostandard() takes the lock but it always drops it before
returning.

This was found with a static checker and compile tested only.  :/

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>