this adds support to axe(4) for the ax88772b usb ethernet adapter
that came with my asus ux21.
it adds the usb device and an ax88772b-specific register that
according to linux, the ax88772b only uses 11 bits of the header
for the actual packet size and the other bits are used for something
else. older chips didn't use those extra 5 bits, so they didn't
produce length mismatches. this makes the length checks only look
at 11 bits.
tested and working on the ax88772b with no input errors. tested and
still working on an older apple ax88772 usb adapter.