Converted from ioctl to unlocked_ioctl.

This driver already used an internal lock, but it was missing in cadet_open and
cadet_release and it was not used correctly in cadet_read.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>