In decode_network_values dstype should be the ordinal value of string literal

I tried using the python network code in contrib to receive data from collectd 5.3.0 and got an exception
arising from decode_network_values.

The problem is that the dstype comparison is actually comparing a string literal with integers to determine the data type. None matches so the code attempts to throw ValueError. There's actually a problem with that too because the exception string uses %i to include the dstype (which, as mentioned, is a string literal).

This patch sets dstype to the ordinal value of the string literal. This fixes the behaviour of the comparisons and in the exception case.