p54: fix broadcast buffering in AP mode

The patch "mac80211: fix PS-poll response race" somehow broke
broadcast buffering in a funny way.

During normal operation - stations are awake - the firmware refused
to transmit broadcast frames and reported P54_TX_PSM_CANCELLED.
But everything worked as soon as one station entered PSM.

The reason:
The stack sets IEEE80211_TX_CTL_SEND_AFTER_DTIM for outgoing
broadcast frames as soon as a station is marked as sleeping.
This flag triggers a path which will reroute these frames
into p54's "content after beacon" queue, which is designed
to cope with the demands for psm.

This patch restores the old behavior.

IEEE80211_TX_CTL_CLEAR_PS_FILT will once again be used to signalize
the firmware to ignore the ps canceling for certain frames.

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