net: dsa: microchip: move vlan functionality to ksz_common

This patch moves the vlan dsa_switch_ops such as vlan_add, vlan_del and
vlan_filtering from the individual files ksz8795.c, ksz9477.c to
ksz_common.c file.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>