ath9k/ath9k_htc: fix txop limit handling

In all those years apparently nobody noticed that the txop limit programmed
into the chip was off by a factor of 32 (!), probably because the VI and VO
queues aren't used that much aside from mgmt frames on VO.

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