V4L/DVB: v4l2-dev: remove unnecessary lock around atomic clear_bit

No need to lock when unregistering the device: clear_bit is already an
atomic operation.

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