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: 1d343579 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Fixes: a8015ded ("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>