CVE-2022-48779 net: mscc: ocelot: fix use-after-free in ocelot_vlan_del()

In the Linux kernel, the following vulnerability has been resolved:

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().