The BCM54810 PHY can use the standard BMCR Power down suspend, but needs
a custom resume routine which first clear the Power down bit, and then
re-initializes the PHY. While in low-power mode, the PHY only accepts
writes to the BMCR register. The datasheet clearly says it:

Reads or writes to any MII register other than MII Control register
(address 00h) while the device is in the standby power-down mode may
cause unpredictable results.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>