net: dsa: mv88e6xxx: add egress mode enumeration

As for the frame mode, add a mv88e6xxx_egress_mode enumeration instead
of a 16-bit register mask.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>