net: dsa: felix: allow the device to be disabled

If there is no specific configuration of the felix switch in the device
tree, but only the default configuration (ie. given by the SoCs dtsi
file), the probe fails because no CPU port has been set. On the other
hand you cannot set a default CPU port because that depends on the
actual board using the switch.

[    2.701300] DSA: tree 0 has no CPU port
[    2.705167] mscc_felix 0000:00:00.5: Failed to register DSA switch: -22
[    2.711844] mscc_felix: probe of 0000:00:00.5 failed with error -22

Thus let the device tree disable this device entirely, like it is also
done with the enetc driver of the same SoC.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>