CHROMIUM: wireless: Add stats for tx latency

Add statistics to measure transmit latency, from packets being enqueued
to wireless driver to tx status returned by firmware being processed by
host driver. The stats is a histogram with exponential scale. Each bin
is a count of packets with latency in that range.

commit 9f6e31cb6b67 ("CHROMIUM: wireless-4.2: Add stats for tx latency.")
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/462403



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

Change-Id: Ifb2cbd53d40d280031ea66d40dfaca8be963b871
Signed-off-by: Ratheesh S P <ratheeshs@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1677132


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>