V4L/DVB (10295): uvcvideo: Retry URB buffers allocation when the system is low on memory.

URB buffers for video transfers are sized to UVC_MAX_PACKETS bulk/isochronous
packets by default. If the system is too low on memory try successively
smaller numbers of packets until allocation succeeds.

Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>