ath9k: Fix TX filter usage

The TX filter bit for a station would be set by the HW
when a frame is not acked. A frame would be completed with
ATH9K_TXERR_FILT status only when the corresponding filter bit
for the destination station is already set.

Currently, un-acknowledged  packets are added to the pending queue
and retried, but the "clear_dest_mask" bit in the descriptor is
set only when the TX status has been ATH9K_TXERR_FILT. This results
in packet loss and the log shows:

wlan0: dropped TX filtered frame, queue_len=0 PS=0 @4309746071
wlan0: dropped TX filtered frame, queue_len=0 PS=0 @4309746076
wlan0: dropped TX filtered frame, queue_len=0 PS=0 @4309746377
...
...

This issue can be resolved by making sure that the destination
mask is cleared when the packet is being retried and the earlier
TX status is ATH9K_TXERR_XRETRY.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>