net: phy: Convert u32 phydev->lp_advertising to linkmode

Convert phy drivers to report the link partner advertised modes using
a linkmode bitmap. This allows them to report the higher speeds which
don't fit in a u32.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>