r8169: remove mii_if_info member from struct rtl8169_private

The only remaining usage of the struct mii_if_info member is to store the
information whether the chip is GMII-capable. So we can replace it with
a simple flag.

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