usbnet: New driver for QinHeng CH9200 devices

There's a bunch of cheap USB 10/100 devices based on QinHeng chipsets. The
vendor driver supports the CH9100 and CH9200 devices, but the majority of
the code is of the if (ch9100) {} else {} form, with the most significant
difference being that CH9200 provides a real MII interface but CH9100 fakes
one with a bunch of global variables and magic commands. I don't have a
CH9100, so it's probably better if someone who does provides an independent
driver for it. In any case, this is a lightly cleaned up version of the
vendor driver with all the CH9100 code dropped.

Signed-off-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: David S. Miller <davem@davemloft.net>