net: phylink: add 1000base-KX to phylink_caps_to_linkmodes()

1000base-KX was missed in phylink_caps_to_linkmodes(), add it. This
will be necessary to convert stmmac with xpcs to ensure we don't drop
any supported linkmodes.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>