dsa: do not dereference non-existing routing table

In the case where there is only one switch, no routing table will have
been allocated, so do not dereference it in this case.
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6131.c

@@ -139,7 +139,8 @@ static int mv88e6131_setup_global(struct dsa_switch *ds)
 		int nexthop;
 
 		nexthop = 0x1f;
-		if (i != ds->index && i < ds->dst->pd->nr_chips)
+		if (ds->pd->rtable &&
+		    i != ds->index && i < ds->dst->pd->nr_chips)
 			nexthop = ds->pd->rtable[i] & 0x1f;
 
 		REG_WRITE(REG_GLOBAL2, 0x06, 0x8000 | (i << 8) | nexthop);