[media] em28xx: radio_fops should also use unlocked_ioctl

em28xx uses core assisted locking, so it shouldn't use .ioctl.
The .ioctl callback was replaced by .unlocked_ioctl for video nodes,
but not for radio nodes. This is now corrected.

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