Changes:

  - match_device_s390_subchannels_parse() should accept un-initialized
    arguments a,b,c, as they are striclty output arguments (without
    transfering ownership).
  - the output arguments should be set if (and only if) the function
    succeeds. That is, move assigning the output arguments to the end.
  - increase the BUFSIZE. It's unclear why choosing 10. Probably that
    was already sufficient as a subchannel looks like
    "0.0.f5f0,0.0.f5f1,0.0.f5f2". Still, increase it to be ample.
    If we want to restrict the parsing based on the lenght of the input,
    that should be done explicitly (but that seems not desirable).
  - use _nm_utils_ascii_str_to_int64() which checks that the range
    of the values fits in guint32.

It seems wrong that match_device_s390_subchannels_eval() only compares
the first of up to three subchannels. But leave it as is for now.

(cherry picked from commit 419151a1)