mac80211: set CLEAR_PS for pspolled frames

This patch sets IEEE80211_TX_CTL_CLEAR_PS_FILT for outgoing
frames for a half-wake station.

this is necessary if one wants to get ps-poll working properly with a p54 ap.

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