SMSC95xx driver updates (round 1)

[Ben Dooks]

This is a series of a few driver cleanups and some fixups of the code
for the SMSC95XX driver. There have been a few reviews, and the issues
have been fixed so this should be ready for merging.

I will work on the tx-alignment and the other bits of usbnet changes
and produce at least two more patch series for this later.