net: dsa: mv88e6xxx: Configure MAC when using fixed link

The 88e6185 is reporting it has detected a PHY, when a port is
connected to an SFP. As a result, the fixed-phy configuration is not
being applied. That then breaks packet transfer, since the port is
reported as being down.

Add additional conditions to check the interface mode, and if it is
fixed always configure the port on link up/down, independent of the
PPU status.

Fixes: 30c4a5b0 ("net: mv88e6xxx: use resolved link config in mac_link_up()")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>