USB ethernet drivers.
usbnet.h introduces a new set of APIs to provide common solutions
for these driver features:
- USB endpoint pipe handling
- rx and tx chain handling
- generic handlers or support for several struct ifnet callbacks
- MII bus locking
- interrupt handling
- partial autoconf handling: much of attach, and detach/activate
can use common versions directly.
currently, only axen(4) and cdce(4) are converted. the reductions
in these drivers are quite significant: if_cdce.c is reduced from
1000 lines to 320 lines, and if_axen is reduced from 1902 lines
to 1021 lines.
add a "usbnet" module and make the if_axen module depend upon it.
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