net: dsa: microchip: update the ksz_port_mdb_add/del

ksz_mdb_add/del in ksz_common.c is specific for the ksz8795.c file. The
ksz9477 has its separate ksz9477_port_mdb_add/del functions.  This patch
moves the ksz8795 specific mdb functionality from ksz_common to ksz8795.
And this dsa_switch_ops hooks for ksz8795/ksz9477 are invoked through
the ksz_port_mdb_add/del.

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