CHROMIUM: wireless: Add pending frames count in latency stats.

Show the number of frames pending in driver for tx latency stats.

commit 42c81098cf11
("CHROMIUM: wireless-4.2: Add pending frames count in latency stats.")
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/545146



BUG=b:122726016
TEST=cat /sys/kernel/debug/ieee80211/phyX/ath10k/stats

Cq-Depend: chromium:1676934
Change-Id: I5fcf695fcda1e58850ac3c8d3984d033243a2c24
Signed-off-by: Ratheesh S P <ratheeshs@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1676935


Tested-by: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Commit-Ready: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Danny Mitzel <mitzel@google.com>
Reviewed-by: Kan Yan <kyan@chromium.org>