net: dsa: sja1105: Avoid error message for unknown PHY mode on disabled ports

When sja1105_init_mii_settings iterates over the port list, it prints
this message for disabled ports, because they don't have a valid
phy-mode:

[    4.778702] sja1105 spi2.0: Unsupported PHY mode unknown!

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Suggested-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>