V4L/DVB (13150): uvcvideo: Don't acquire privileges in VIDIOC_TRY_FMT

The VIDIOC_TRY_FMT ioctl doesn't interfere with video streaming and thus
should succeed on an unprivileged file handle.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>