brcmfmac: fix missing unlock on error in brcmf_notify_vif_event()

Add the missing unlock before return from function brcmf_notify_vif_event()
in the error handling case.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.c

@@ -4615,8 +4615,10 @@ static s32 brcmf_notify_vif_event(struct brcmf_if *ifp,
 	switch (ifevent->action) {
 	case BRCMF_E_IF_ADD:
 		/* waiting process may have timed out */
-		if (!cfg->vif_event.vif)
+		if (!cfg->vif_event.vif) {
+			mutex_unlock(&event->vif_event_lock);
 			return -EBADF;
+		}
 
 		ifp->vif = vif;
 		vif->ifp = ifp;