net: dsa: lan9303: Add adjust_link() method

Make the driver react to device tree "fixed-link" declaration on CPU port.

- turn off autonegotiation
- force speed 10 or 100 mb/s
- force duplex mode

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: David S. Miller <davem@davemloft.net>