mac80211: ignore PSM bit of reordered frames

This patch tackles one of the problems of my
reorder release timer patch from August.

<http://www.spinics.net/lists/linux-wireless/msg57214.html

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What if the reorder release triggers and ap_sta_ps_end
(called by ieee80211_rx_h_sta_process) accidentally clears
the WLAN_STA_PS_STA flag, because 100ms ago - when the STA
was still active - frames were put into the reorder buffer.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>