ath10k: Added support to reset HTT stats in debugfs

Reset HTT stats helps to get the aggregated HTT stats via
tracing and also we can clear the accumulated HTT stats with
this debugfs file.

Signed-off-by: Maharaja Kennadyrajan <mkenna@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>