[media] uvc gadget: switch to unlocked_ioctl

Instead of .ioctl use unlocked_ioctl. This allows us to finally remove
the old .ioctl op.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>