Setting mac_managed_pm during interface up is too late.

In situations where the link is not brought up yet and the system suspends
the regular PHY power management will run. Since the FEC ETHEREN control
bit is cleared (automatically) on suspend the controller is off in resume.
When the regular PHY power management resume path runs in this context it
will write to the MII_DATA register but nothing will be transmitted on the
MDIO bus.

This can be observed by the following log:

    fec 5b040000.ethernet eth0: MDIO read timeout
    Microchip LAN87xx T1 5b040000.ethernet-1:04: PM: dpm_run_callback(): mdio_bus_phy_resume+0x0/0xc8 returns -110
    Microchip LAN87xx T1 5b040000.ethernet-1:04: PM: failed to resume: error -110

The data written will however remain in the MII_DATA register.

When the link later is set to administrative up it will trigger a call to
fec_restart() which will restore the MII_SPEED register. This triggers the
quirk explained in f166f890 ("net: ethernet: fec: Replace interrupt
driven MDIO with polled IO") causing an extra MII_EVENT.

This extra event desynchronizes all the MDIO register reads, causing them
to complete too early. Leading all reads to read as 0 because
fec_enet_mdio_wait() returns too early.

When a Microchip LAN8700R PHY is connected to the FEC, the 0 reads causes
the PHY to be initialized incorrectly and the PHY will not transmit any
ethernet signal in this state. It cannot be brought out of this state
without a power cycle of the PHY.

Fixes: 557d5dc8 ("net: fec: use mac-managed PHY PM")
Closes: https://lore.kernel.org/netdev/1f45bdbe-eab1-4e59-8f24-add177590d27@actia.se/Signed-off-by: Wei Fang <wei.fang@nxp.com>
[jernberg: commit message]
Signed-off-by: John Ernberg <john.ernberg@actia.se>
Link: https://lore.kernel.org/r/20240328155909.59613-2-john.ernberg@actia.seSigned-off-by: Jakub Kicinski <kuba@kernel.org>