mac80211: dynamic PS - don't enter PS when TX frames are pending

mac80211: dynamic PS - don't enter PS when TX frames are pending

Use the tx_frames_pending() driver callback to determine if Tx frames are
pending for its internal queues. If so postpone the dynamic PS timeout
to avoid interrupting Tx traffic.

The commit ff71bee1502a531189f8746ca74c0de0ea06f06f enabled this
behavior for drivers with IEEE80211_HW_PS_NULLFUNC_STACK. We enable this
for all drivers supporting dynamic PS.

This patch helps improve performance in noisy environments.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>