wifi: ath9k: avoid using uninitialized array

In 'ath_tx_count_frames()', 'ba' array may be used uninitialized, so
add 'memset()' call similar to one used in 'ath_tx_complete_aggr()'.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230620080855.396851-1-dmantipov@yandex.ru