net: dsa: mv88e6xxx: clarify SMI PHY functions

Marvell chips with an SMI PHY access in Global 2 registers handle both
Clause 22 and Clause 45 of IEEE 802.3.

The 88E6390 family has addition bits to target the internal or external
PHYs connected to the device, and a Setup function in addition to the
default (register) Access function.

Prefix the SMI PHY Command and Data registers macros, implement clear
helpers for Clause 22 and 44 Access functions, rename variable to match
the SMI and switch vocabulary (device and register addresses for Clause
22 and port and device class for Clause 45.)

Finally do not use complex macros but simple 16-bit mask to document the
registers organization.

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