This is a little change over a lot of files. It changes the
return value of network device transmit from an integer to
an enum type (like irqreturn_t).  This allows compiler to warn
about stupid code that tries to return -ENOBUFS or other mistakes
like that. All the code that did that is gone, but it would
be better to try and do some decent type checking.

This patch converts all the x86 drivers (except staging).
Other drivers will work but produce a warning until converted.

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