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 e8306f98 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>