ath10k: protect wep tx key setup

All configuration sequences should be protected
with conf_mutex to avoid concurrent/conflicting
requests.

This should make sure that wep tx key setup is not
performed while hw is restarted (at least).

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