net: dsa: mv88e6xxx: Single chip mode detection for MV88E6*41

The mv88e6xxx driver expects switches that are configured in single chip
addressing mode to have the MDIO address configured as 0. This is due to
the switch ADDR pins representing the single chip addressing mode as 0.
However depending on the device (e.g. MV88E6*41) the switch does not
respond on address 0 or any other address below 16 (the first port
address) in single chip addressing mode. This allows for other devices
to be on the same shared MDIO bus despite the switch being in single
chip addressing mode.

When using a switch that works this way it is not possible to configure
switch driver as single chip addressing via device tree, along with
another MDIO device on the same bus with address 0, as both devices
would have the same address of 0 resulting in mdiobus_register_device
-EBUSY errors for one of the devices with address 0.

In order to support this configuration the switch node can have its MDIO
address configured as 16 (the first address that the device responds
to). During initialization the driver will treat this address similar to
how address 0 is, however because this address is also a valid
multi-chip address (in certain switch models, but not all) the driver
will configure the SMI in single chip addressing mode and attempt to
detect the switch model. If the device is configured in single chip
addressing mode this will succeed and the initialization process can
continue. If it fails to detect a valid model this is because the switch
model register is not a valid register when in multi-chip mode, it will
then fall back to the existing SMI initialization process using the MDIO
address as the multi-chip mode address.

This detection method is safe if the device is in either mode because
the single chip addressing mode read is a direct SMI/MDIO read operation
and has no side effects compared to the SMI writes required for the
multi-chip addressing mode.

In order to implement this change, the reset gpio configuration is moved
to occur before any SMI initialization. This ensures that the device has
the same/correct reset gpio state for both mv88e6xxx_smi_init calls.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220427130928.540007-1-nathan@nathanrossi.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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