athn(4) usb (GSoC)

Hi Martin,

I realized taking the athn_softc out wouldn't be so simple because of
the common code in arn*.c.  So I tried firstly without usbwifi (on a
branch on the git repository, and
https://jonathan50.github.io/gsoc-blog/athnusb1.patch).  If it doesn't
work (completely untested, of course, since I don't have the hardware)
then hopefully it'll be easy for someone to fix...

Then I tried an approach with usbwifi, changing sc_ic to a pointer
(probably not the best way to go about it):
https://jonathan50.github.io/gsoc-blog/athnusb2.patch  Less confident
this one will work (though it compiled and PCI athn(4) still works).
It probably should, if I've done the right thing with the USB pipes --
one is still maintained by the driver for commands.  That also adds a
length parameter to uwo_intr (no other driver uses uwo_intr yet).

I suppose it would be much better to make a struct with all the common
data, and make all the functions of arn*.c use that instead of the
softc.  Then sc_ic would be left undisturbed (as in every other
driver).  So I could try that next.

But I will possibly get hold of an Atheros USB dongle in about a week,
and then I can test it myself.

Jeandre