usbnet private-data accessor functions

[Ben Dooks]

I have been looking at the usbnet drivers and the possibility of some
code cleanups. One of the things I've found is that changing the way
the drivers use the private data with the usbnet structure is often
hand-coded each time is needed.

An easy cleanp (and making it easier in the future to change the
access method) would be to add an inline conversion function to
each driver so that it is done in one place.

Future work would be to look at the usbnet.data and see if it could
be changed (such as moving to a union, allocation of the data after
the usbnet structure, or similar).