On PHY state change the phylink_resolve() function can read stale
information from the MAC and report incorrect link speed and duplex to
the kernel message log.

Example with a Marvell 88X3310 PHY connected to a SerDes port on Marvell
88E6393X switch:
- PHY driver triggers state change due to PHY interface mode being
  changed from 10gbase-r to 2500base-x due to copper change in speed
  from 10Gbps to 2.5Gbps, but the PHY itself either hasn't yet changed
  its interface to the host, or the interrupt about loss of SerDes link
  hadn't arrived yet (there can be a delay of several milliseconds for
  this), so we still think that the 10gbase-r mode is up
- phylink_resolve()
  - phylink_mac_pcs_get_state()
    - this fills in speed=10g link=up
  - interface mode is updated to 2500base-x but speed is left at 10Gbps
  - phylink_major_config()
    - interface is changed to 2500base-x
  - phylink_link_up()
    - mv88e6xxx_mac_link_up()
      - .port_set_speed_duplex()
        - speed is set to 10Gbps
    - reports "Link is Up - 10Gbps/Full" to dmesg

Afterwards when the interrupt finally arrives for mv88e6xxx, another
resolve is forced in which we get the correct speed from
phylink_mac_pcs_get_state(), but since the interface is not being
changed anymore, we don't call phylink_major_config() but only
phylink_mac_config(), which does not set speed/duplex anymore.

To fix this, we need to force the link down and trigger another resolve
on PHY interface change event.

Fixes: 9525ae83 ("phylink: add phylink infrastructure")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>