ath10k: clear tx status before submitting to mac80211

Garbage was reported in ieee80211_tx_info. This
led to a WARN_ON in cfg80211_calculate_bitrate().

This also fixes some random tx bitrate values
reported through `iw` command.

Reported-By: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>