r8169: don't set min_mtu/max_mtu if not needed

Defaults for min_mtu and max_mtu are set by ether_setup(), which is
called from devm_alloc_etherdev(). Let rtl_jumbo_max() only return
a positive value if actually jumbo packets are supported. This also
allows to remove constant Jumbo_1K which is a little misleading anyway.

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