Previously, PHY addressing was a concern of the `Ethernet` struct
which limited the `PHY` implementations which very often have to manage
multiple PHYs internally and thus possibly need to address many of them.
This change extends `StationManagement` to allow addressing different
PHY addresses via SMI.
SMI Ethernet PHYs all share a common base set of registers that can do
90% of all tasks. The LAN8742 driver used some vendor-specific
registers to check link negotiation status, but the need for that was
debatable, so I migrated it to a generic driver instead, anybody who
wants extra functionality can copy it and impl their own on top of it.
