ath9k: another fix for the A-MPDU buffer leak

The patch 'ath9k: fix a buffer leak in A-MPDU completion' addressed the
issue of running out of buffers/descriptors in the tx path if a STA is
deleted while tx status feedback is still pending.
The remaining issue is that the skbs of the buffers are not reclaimed,
leaving a memory leak.
This patch fixes this issue by running the buffers through
ath_tx_complete_buf(), ensuring that the pending frames counter is also
updated.

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