net: dsa: mv88e6xxx: correctly report serdes link failure

Phylink wants to know if the link has dropped since the last time state
was retrieved, and the BMSR gives us that. Read the BMSR and use it when
deciding the link state. Fill in the an_complete member as well for the
emulated PHY state.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>