mac80211: FIF_PSPOLL filter flag

When an interface is configured in the AP mode, the mac80211
implementation doesn't inform the driver to receive PS Poll frames.
It leads to inability to communicate with power-saving stations
reliably.
The FIF_CONTROL flag isn't passed by mac80211 to
ieee80211_ops.configure_filter when an interface is in the AP mode.
And it's ok, because we don't want to receive ACK frames and other
control ones, but only PS Poll ones.

This patch introduces the FIF_PSPOLL filter flag in addition to
FIF_CONTROL, which means for the driver "pass PS Poll frames".

This flag is passed to the driver:
A) When an interface is configured in the AP mode.
B) In all cases, when the FIF_CONTROL flag was passed earlier (in
addition to it).

Signed-off-by: Igor Perminov <igor.perminov@inbox.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>