Simple conversion with a bit of needed cleanup.

This also fixes:
 - confusion around vlan_features in rtl8169_vlan_mode(),
 - problem with broken TSO for too big MTU (the limit is set
   at 0xFFF --- max MSS field value).

SG+IP_CSUM+TSO is left disabled by default, based on suggestion by
David Dillow.
Signed-off-by: NMichał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>