iwlwifi: mvm: use atomic interface iteration to avoid deadlock

Using the non-atomic version creates a dependency between
mac80211's iflist_mtx and mvm->mutex. Use the atomic version
instead which doesn't take iflist_mtx but can't sleep, so
send the HCMD in ASYNC.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>