ath10k: fix NULL deref upon early FW crash

If firmware crashes during FW probing it would try
to perform FW recovery which uses mac80211
workqueue before registering to mac80211.

Using internal workqueue solves the problem.

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