[Nut-upsdev] Change required for upscode2 driver

[Arjen de Korte]

Niklas, Niels,

Some changes are required in the upscode2 driver since ser_get_line()  
will now return 0 on timeouts. This means that there is no way to tell  
this apart from an empty line, which is a condition that is checked  
for in this driver. I committed a change to prevent the driver from  
locking up on timeouts, but this removed the empty line skipping in  
the process.

Note that using ser_get_line() to discard empty lines is (and never  
was) a good way to do this. The function internally reads up to 64  
bytes at a time and if the first byte is equal to ENDCHAR, the  
remaining characters are discarded and lost forever. See also the  
example in docs/new-drivers.txt. In all likelihood, this never worked  
like intended.

Best regards, Arjen
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