[media] uvcvideo: Enable VIDIOC_CREATE_BUFS

This patch enables the ioctl to create additional buffers on the
videobuf2 capture queue.

[laurent.pinchart@ideasonboard.com: Acquire privileges instead of just
checking them in VIDIOC_CREATE_BUFS implementation]

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>