net: renesas: rswitch: Convert to phy_device

Intended to set phy_device->host_interfaces by phylink in the future.
But there is difficult to implement phylink properly, especially
supporting the in-band mode on this driver because extra initialization
is needed after linked the ethernet PHY up. So, convert to phy_device
from phylink.

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