CHROMIUM: ath10k: Fix array overflow in debug.c

This CL fixes an array overflow introduced during manual port of
chromium:541625 to v4.14-gw in drivers/net/wireless/ath/ath10k/debug.c

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

Change-Id: If555ff8693b41d8945f066979f3d5a8b3e9774b3
Signed-off-by: Omkar Bahirat <omkarvijay@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1700428


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