[media] uvcvideo: Rename uvc_alloc_buffers to uvc_request_buffers

This brings the function name in line with the V4L2 API terminology.

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