ath10k: release conf_mutex if vdev_start() fails

I modified Michal's commit c930f744 ("ath10k: implement channel switching") to return
when vdev_start() fails, but forgot to release conf_mutex.

Found by coccinelle:

>> drivers/net/wireless/ath/ath10k/mac.c:2745:5-11: preceding lock on line 2663
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath10k/mac.c

@@ -2785,7 +2785,7 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
 				if (ret) {
 					ath10k_warn("failed to start vdev: %d\n",
 						    ret);
-					return;
+					goto exit;
 				}
 
 				arvif->is_started = true;
@@ -2866,6 +2866,7 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
 			ath10k_bss_assoc(hw, vif, info);
 	}
 
+exit:
 	mutex_unlock(&ar->conf_mutex);
 }