iwlwifi: mvm: use helpers to get iwl_mvm_sta

Getting the mvm station out of station id requires dereferencing
the station id to get ieee80211_sta, then checking for pointer
validity and only then extract mvm station out.
Given that there are helpers to do it - use them instead of
duplicating the code whenever we need only mvm station.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
diff --git a/drivers/net/wireless/intel/iwlwifi/mvm/debugfs-vif.c b/drivers/net/wireless/intel/iwlwifi/mvm/debugfs-vif.c
index fb96bc0..b232717 100644
--- a/drivers/net/wireless/intel/iwlwifi/mvm/debugfs-vif.c
+++ b/drivers/net/wireless/intel/iwlwifi/mvm/debugfs-vif.c
@@ -281,13 +281,10 @@
 
 	if (vif->type == NL80211_IFTYPE_STATION &&
 	    ap_sta_id != IWL_MVM_STATION_COUNT) {
-		struct ieee80211_sta *sta;
+		struct iwl_mvm_sta *mvm_sta;
 
-		sta = rcu_dereference_protected(mvm->fw_id_to_mac_id[ap_sta_id],
-						lockdep_is_held(&mvm->mutex));
-		if (!IS_ERR_OR_NULL(sta)) {
-			struct iwl_mvm_sta *mvm_sta = iwl_mvm_sta_from_mac80211(sta);
-
+		mvm_sta = iwl_mvm_sta_from_staid_protected(mvm, ap_sta_id);
+		if (mvm_sta) {
 			pos += scnprintf(buf+pos, bufsz-pos,
 					 "ap_sta_id %d - reduced Tx power %d\n",
 					 ap_sta_id,