ath9k: improve PS filter clearing and retry counting for A-MPDU

Do not increment the retry counter if packets to a sleeping station
were not sent because of tx failure, instead of only checking the filter
flag.
Clear the PS filter only after an A-MPDU was reported as filtered,
otherwise the hardware might do some unnecessary extra retransmissions.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>