net: dsa: microchip: move the port mirror to ksz_common

This patch updates the common port mirror add/del dsa_switch_ops in
ksz_common.c. The individual switches implementation is executed based
on the ksz_dev_ops function pointers.

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