ravb: Do not announce HDX as supported

According to the data sheet the Ethernet-AVB hardware in R-Car Gen3
and R-Car Gen2 SoCs do not support half duplex operation. So update
the driver to mark 100Mbit and 1Gbps HDX as unsupported.

Fixes: c156633f ("Renesas Ethernet AVB driver proper")
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>