CHROMIUM: ath10k: Add max number of pending frames to latency stats.

Manually ported from chromeos-kernel-3_18 commit
24b07ff1ed35fcb6e8649d2b1fbb0b8664830314

BUG=b:137048469
TEST="cat /sys/kernel/debug/ieee80211/phy1/ath10k/tx_delay_histogram/stats"
      expected first line of the output:
      Frames pending in driver: <count>, max: <count>

Change-Id: Ic63c34a15368876aa77c76335185789ec5dd3240
Signed-off-by: Omkar Bahirat <omkarvijay@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1692330


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Kan Yan <kyan@chromium.org>