usbnet transmit scatter-gather

usbnet and ax88179_178a.c where changed fairly recently to support
scatter-gather transmit.
(This would let TSO be re-enabled.)

The change removed the skb_serialize() from ax88179_178a.c.
This means that things will go horribly wrong if the hardware
doesn't support SG (my i7 box must support SG), I'm sure
I can find something that doesn't :-)

I think the interface to usbnet needs changing so that:
1) The driver reports whether it can process segmented skb.
2) If the driver can't process segmented skb, usbnet will
   call skb_serialize() prior to calling tx_fixup().
3) If the usb hardware can't do SG, usbnet will call
   skb_serialize() after calling tx_fixup().

Would it make sense for usbnet mask out TSO support
when SG dma is unavailable?

	David