net: mii: add linkmode_adv_to_mii_adv_x()

Add a helper to convert a linkmode advertisement to a clause 37
advertisement value for 1000base-x and 2500base-x.

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>