net: dsa: microchip: move get_phy_flags & mtu to ksz_common

This patch assigns the get_phy_flags & mtu  hook of ksz8795 and ksz9477
in dsa_switch_ops to ksz_common.  For get_phy_flags hooks,checks whether
the chip is ksz8863/kss8793 then it returns error for port1.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>