r8169: Config1 is read-only on 8168c and later.

commit 851e60221926a53344b4227879858bef841b0477 upstream.

Suggested by Hayes.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Hayes Wang <hayeswang@realtek.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>