V4L/DVB (11231): hdpvr: locking fixes

unlock io_mutex in hdpvr_stop_streaming hdpvr_disconnect to allow the
streaming worker to stop before we flush the workqueue.
do not return to user space with mutex held in vidioc_encoder_cmd with
an unknown encoder command.

Signed-off-by: Janne Grunau <j@jannau.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>