net: mscc: ocelot: fix use-after-free in ocelot_vlan_del()

ocelot_vlan_member_del() will free the struct ocelot_bridge_vlan, so if
this is the same as the port's pvid_vlan which we access afterwards,
what we're accessing is freed memory.

Fix the bug by determining whether to clear ocelot_port->pvid_vlan prior
to calling ocelot_vlan_member_del().

Fixes: d4004422 ("net: mscc: ocelot: track the port pvid using a pointer")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>