ath9k: fix queue depth accounting in ath_tx_txqaddbuf

ath_tx_txqaddbuf assumes that all the linked buffers in the queue passed
to it are part of the same A-MPDU or MPDU. The CAB queue rework violates
this assumption, which can cause the internal queue depth to go
negative.
Fix this by increasing the counter for all slots of [bf, bf->bf_lastbf]

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