ath10k: clear htt.freq

If htt.freq isn't cleared and contains garbage fw
may discard tx packets. Prevent this from
happening by clearing htt.freq properly.

Possible manifestation of the problem could be not
being able to send auth request/response frames on
firmware with HTT >= 3.4 (when freq param was
introduced), e.g. on qca6174.

Fixes: 8d6d3624 ("ath10k: fix offchan reliability")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>