USB GeneLink/NDIS tweaks

This patch, from Oliver, discards slightly less data in cases
where memory is very tight.  The GeneLink framing can batch
several packets in one transmission, so it's always possible to
at least report the last one, since it's already sitting in an SKB
and we won't need a new one.

Random note, this seems to be the same issue I saw
mentioned somewhere else recently:  MSFT host drivers
with high latency for submitting bulk packets, so rather
than just queuing them as they arrive, it turns out to be a
win on the MSFT side to batch packets.