ath9k: don't strip mic on non-encrypted frames in tkip

Fix the following bug: in tkip mode, qos-null ps on/off packets
are dropped due to incorrect packet length so that ath9k softap
can't handle powersave state transition of peer STA correctly.

Signed-off-by: Michael Liang <mliang@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>