mac80211: clear more-data bit on filtered frames

It doesn't seem likely, but maybe possible, that the
more-data bit needs to be recomputed due to changes
in the queued frames. Clear it for filtered frames
to ensure that we never send it incorrectly. It'll
be set again as necessary when we retransmit this
frame.

The more likely case is maybe where the station woke
up after the filtered frame in which case more-data
should be clear when the frame is transmitted to the
station since it is now awake.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>