#3 fix for Siemens S7 MPI drivers

[Thomas Hergenhahn]

I changed the function _daveTestChars(), so that it
can handle single bytes in the mid of a sequence as
don´t care. This was a quick way to use the same code
for S7-300 and 400 CPUs. I merged in this function
from what I had on my notebook, but obviously didn´t
test properly.
This is a quick fix.
Replaces the file nodave.c in directory drivers.