[patch] Making tulip.c work on linuxppc

I noticed in the comments to 0.91 that it claimed to support powerpc on
"all but <one early 21x4x> which does not have native mixed-endian
support".  Well, I've gone through two tulip clones (a PNIC and my
current Macronix 98713) which apparently don't either.  So, I made
tulip work in little-endian mode for these.

I don't know how this patch would interact with a card which has this
fabled mixed-endian support -- probably very badly.  But for my two, it
worked.  It also cleans up a few warnings that I noticed while working
- one of the comparison between signed and unsigned turned out to make
a significant difference).

Thoughts, anyone?

Dan

/--------------------------------\  /--------------------------------\
|       Daniel Jacobowitz        |__|        SCS Class of 2002       |
|   Debian GNU/Linux Developer    __    Carnegie Mellon University   |
|         dan@debian.org         |  |       dmj+@andrew.cmu.edu      |
\--------------------------------/  \--------------------------------/