net: dsa: mv88e6xxx: prefix Port Switch ID macros

For implicit namespacing and clarity, prefix the common Switch ID
Register macros with MV88E6XXX_PORT_SWITCH_ID.

Document the register and prefer ordered hex masks values for all
Marvell 16-bit registers, this means shifting their values by 4.

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