BACKPORT: ath9k: Switch to mac80211 TXQ scheduling and airtime APIs

This moves the ath9k driver to use the mac80211 TXQ scheduling and
airtime accounting APIs, removing the corresponding state tracking
inside the driver.

Signed-off-by: Toke Hiland-Jrgensen <toke@toke.dk>
[rmanohar@codeaurora.org: fixed checkpatch error and warnings]
Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

BUG=b:131197056
TEST=build 4.14-gw kernel
(cherry picked from commit 89cea749)

Signed-off-by: Kan Yan <kyan@google.com>
Change-Id: I10f4bc633d5052ff7b188b56b5dcac2f2bb1bf3b
Reviewed-on: https://chromium-review.googlesource.com/1637432


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Julan Hsu <julanhsu@google.com>
Reviewed-by: Kan Yan <kyan@chromium.org>