net: xpcs: add support for retrieving supported interface modes

Add a function to the xpcs driver to retrieve the supported PHY
interface modes, which can be used by drivers to fill in phylink's
supported_interfaces mask.

We validate the interface bit index to ensure that it fits within the
bitmap as xpcs lists PHY_INTERFACE_MODE_MAX in an entry.

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>