brcmfmac: remove monitor interface when detaching

This fixes a minor WARNING in the cfg80211:
[  130.658034] ------------[ cut here ]------------
[  130.662805] WARNING: CPU: 1 PID: 610 at net/wireless/core.c:954 wiphy_unregister+0xb4/0x198 [cfg80211]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c

@@ -1350,6 +1350,11 @@ void brcmf_detach(struct device *dev)
 	brcmf_fweh_detach(drvr);
 	brcmf_proto_detach(drvr);
 
+	if (drvr->mon_if) {
+		brcmf_net_detach(drvr->mon_if->ndev, false);
+		drvr->mon_if = NULL;
+	}
+
 	/* make sure primary interface removed last */
 	for (i = BRCMF_MAX_IFS - 1; i > -1; i--) {
 		if (drvr->iflist[i])