net: renesas: rswitch: Add "max-speed" handling

The previous code set the speed by the interface mode of PHY.
Also this hardware has a restriction which cannot change the speed
at runtime. To use other speed, add "max-speed" handling to set
each port's speed if needed.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>