net: dsa: drop vlan_getnext

The VLAN GetNext operation is specific to some switches, and thus can be
complicated to implement for some drivers.

Remove the support for the vlan_getnext/port_pvid_get approach in favor
of the generic and simpler port_vlan_dump function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>