[ipoib] Simplify test for received broadcast packets

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/ipoib.c

 	ethhdr->h_protocol = net_proto;
 
 	/* Construct destination address */
-	if ( dest->gid_present &&
-	     ( memcmp ( &dest->gid, &ipoib->broadcast.mac.gid,
-			sizeof ( dest->gid ) ) == 0 ) ) {
-		/* Broadcast GID; use the Ethernet broadcast address */
+	if ( IB_LID_MULTICAST ( dest->lid ) ) {
+		/* Multicast LID; use the Ethernet broadcast address */
 		memcpy ( &ethhdr->h_dest, eth_broadcast,
 			 sizeof ( ethhdr->h_dest ) );
 	} else {