zd1211rw: fix ack_pending in filter_ack causing tx-packet ordering problem on monitor

For reasons not very clear yet to me, filter_ack leaves matching tx-packet
pending with 'ack_pending'. This causes tx-packet to be passed back to upper
layer after next packet has been transfered and tx-packets might end up
coming come out of monitor interface in wrong order vs. rx.

Because of this when enable AP-mode, hostapd monitor interface would get
packets in wrong order causing problems in WPA association.

So don't use mac->ack_pending when in AP-mode.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>