net: dsa: ocelot: populate supported_interfaces

Populate the supported interfaces bitmap for the Ocelot DSA switches.

Since all sub-drivers only support a single interface mode, defined by
ocelot_port->phy_mode, we can handle this in the main driver code
without reference to the sub-driver.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>