libobs: Fix force mono channel count

I unintentionally made it use obs_source::sample_info instead of using
the actual target channel count, which is designated by the OBS output
sampler info.  obs_source::sample_info is actually used to indicate the
currently set sampler information for incoming samples.  So if a source
is outputting 5.1 channel 48khz audio, and OBS is running at stereo
44.1khz, then the obs_source::sample_info value would be set to
5.1/48khz, not the other way around.  It indicates what the source
itself is running at, not what OBS is running at.

I suppose the variable needs a better name because even I used it
incorrectly despite actually having been the one who wrote it.