net: phy: add helper linkmode_adv_to_mii_10gbt_adv_t

Add a helper linkmode_adv_to_mii_10gbt_adv_t(), similar to
linkmode_adv_to_mii_adv_t.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>