net: dsa: mv88e6xxx: fix Serdes link changes

phylink_mac_change() is supposed to be called with a 'false' argument
if the link has gone down since it was last reported up; this is to
ensure that link events along with renegotiation events are always
correctly reported to userspace.

Read the BMSR once when we have an interrupt, and report the link
latched status to phylink via phylink_mac_change().  phylink will deal
automatically with re-reading the link state once it has processed the
link-down event.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>