net: phy: simplify genphy_config_advert by using the linkmode_adv_to_xxx_t functions

Using linkmode_adv_to_mii_adv_t and linkmode_adv_to_mii_ctrl1000_t
allows to simplify the code. In addition avoiding the conversion to
the legacy u32 advertisement format allows to remove the warning.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>