net: dsa: mv88e6xxx: prefix Port Default VLAN macros

For implicit namespacing and clarity, prefix the common Port Default
VLAN Register macros with MV88E6XXX_PORT_DEFAULT_VLAN.

Document the register and prefer ordered hex masks values for all
Marvell 16-bit registers.

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