ravb: Clean up duplex handling

Since only full-duplex operation is supported by the
hardware, remove duplex handling code and keep the
register setting of ECMR.DM fixed at 1.

This updates the driver implementation to follow the
data sheet text "This bit should always be set to 1."

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>