net: dsa: sja1105: Reject unsupported link modes for AN

Ethernet flow control:

The switch MAC does not consume, nor does it emit pause frames. It
simply forwards them as any other Ethernet frame (and since the DMAC is,
per IEEE spec, 01-80-C2-00-00-01, it means they are filtered as
link-local traffic and forwarded to the CPU, which can't do anything
useful with them).

Duplex:

There is no duplex setting in the SJA1105 MAC. It is known to forward
traffic at line rate on the same port in both directions. Therefore it
must be that it only supports full duplex.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>