net: mii: convert mii_lpa_to_ethtool_lpa_x() to linkmode variant

Add a LPA to linkmode decoder for 1000BASE-X protocols; this decoder
only provides the modify semantics similar to other such decoders.
This replaces the unused mii_lpa_to_ethtool_lpa_x() helper.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>