[9fans] fs kernel ether83815 driver updated

[geoff]

I've just updated the 83815 driver in the fs kernel to match the
latest corresponding driver in the cpu kernel.  I haven't tested it
yet, but my old fs driver was somewhat crufty and it looks like
someone (Charles?) has in the latest cpu driver cleaned up some of the
mess I made to support the SiS variants.  If you try the new driver
and it gives you grief, let me know and fall back.