net: phylink: add helper function to decode USXGMII word

With the new addition of the USXGMII link partner ability constants we
can now introduce a phylink helper that decodes the USXGMII word and
populates the appropriate fields in the phylink_link_state structure
based on them.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>