Some switch has dedicated SMI PHY Command and Data registers, used to
indirectly access the PHYs, instead of direct access.

Identify these switch models and make mv88e6xxx_phy_{read,write} generic
enough to support every models.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>