iwlwifi: mvm: provide helper to fetch the iwl_mvm_sta from sta_id

We somtimes need to fetch the iwl_mvm_sta structure from a
station index - provide a helper to do that.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/mvm/bt-coex.c b/drivers/net/wireless/iwlwifi/mvm/bt-coex.c
index 76cde6c..b1a572e 100644
--- a/drivers/net/wireless/iwlwifi/mvm/bt-coex.c
+++ b/drivers/net/wireless/iwlwifi/mvm/bt-coex.c
@@ -500,23 +500,13 @@
 		.dataflags = { IWL_HCMD_DFL_DUP, },
 		.flags = CMD_ASYNC,
 	};
-
-	struct ieee80211_sta *sta;
 	struct iwl_mvm_sta *mvmsta;
 	int ret;
 
-	if (sta_id == IWL_MVM_STATION_COUNT)
+	mvmsta = iwl_mvm_sta_from_staid_protected(mvm, sta_id);
+	if (!mvmsta)
 		return 0;
 
-	sta = rcu_dereference_protected(mvm->fw_id_to_mac_id[sta_id],
-					lockdep_is_held(&mvm->mutex));
-
-	/* This can happen if the station has been removed right now */
-	if (IS_ERR_OR_NULL(sta))
-		return 0;
-
-	mvmsta = iwl_mvm_sta_from_mac80211(sta);
-
 	/* nothing to do */
 	if (mvmsta->bt_reduced_txpower == enable)
 		return 0;