net: dsa: mv88e6xxx: add set_switch_mac to ops

Add a set_switch_mac chip-wide function to mv88e6xxx_ops and remove
MV88E6XXX_FLAG_G2_SWITCH_MAC flags.

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