brcmfmac: bail out of brcmf_txflowblock_if() for non-netdev interface

To avoid ending up in a NULL-pointer access, the function
brcmf_txflowblock_if() should only be called for interfaces
that have a netdev associated with it.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -242,7 +242,7 @@ void brcmf_txflowblock_if(struct brcmf_if *ifp,
 {
 	unsigned long flags;
 
-	if (!ifp)
+	if (!ifp || !ifp->ndev)
 		return;
 
 	brcmf_dbg(TRACE, "enter: idx=%d stop=0x%X reason=%d state=%d\n",