media: uvcvideo: Utilise for_each_uvc_urb iterator

A new iterator is available for processing UVC URB structures. This
simplifies the processing of the internal stream data.

Convert the manual loop iterators to the new helper, adding an index
helper to keep the existing debug print.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>