mwl8k: use traffic threshold to decide when to start ampdu

Currently, ampdu stream is created on the first qos packet to an
HT sta. The overhead of setting up the BA session may not be
justified if the outgoing packet rate is minimal (e.g., ping). So
we only allow ampdu streams after seeing a critical number of
packets in an arbitrary one-second interval.

Based on work by Nishant Sarmukadam <nishants@marvell.com>

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>