net: mscc: ocelot: adjust maxlen on NPI port, not CPU

Being a non-physical port, the CPU port does not have an ocelot_port
structure, so the ocelot_port_writel call inside the
ocelot_port_set_maxlen() function would access data behind a NULL
pointer.

This is a patch for net-next only, the net tree boots fine, the bug was
introduced during the net -> net-next merge.

Fixes: 1d34357 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Fixes: a8015de ("net: mscc: ocelot: properly account for VLAN header length when setting MRU")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mscc/ocelot.c

@@ -2341,7 +2341,7 @@ void ocelot_configure_cpu(struct ocelot *ocelot, int npi,
 		else if (injection == OCELOT_TAG_PREFIX_LONG)
 			sdu += OCELOT_LONG_PREFIX_LEN;
 
-		ocelot_port_set_maxlen(ocelot, cpu, sdu);
+		ocelot_port_set_maxlen(ocelot, npi, sdu);
 
 		/* Enable NPI port */
 		ocelot_write_rix(ocelot,