GPIB overhaul, part #2: make the tnt4882 driver work with the newer
TNT5004 IC.  This involved a major rewrite of a number of things, as
this chip no longer supports the NAT7210 legacy mode but requires the
host to use the (more modern) FIFO mode.

In theory, this also ought to work on the older TNT4882C chip.  I'll
probably add this as optional support (perhaps by a device.hints flag)
later on.  By now, FIFO mode is *only* activates iff a TNT5004 chip
has been detected (where the old code didn't work at all), while
everything else is supposed to use the old code.