net: stmmac: fill in supported_interfaces

Fill in phylink's supported_interfaces bitmap with the PHY interface
modes which can be used to talk to the PHY.

We indicate that the PHY interface mode passed in platform data is
always supported, as this is the initial mode passed into phylink.
When there is no PCS specified, we assume that this is the only mode
that is supported - indeed, the driver appears not to support dynamic
switching of interface types at present.

When a xpcs is present, it defines the PHY interface modes that the
stmmac driver can support. Request the supported interfaces from the
xpcs driver, and pass them to phylink.

Tested-by: Wong Vee Khee &lt;vee.khee.wong@linux.intel.com&gt; # Intel EHL Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>