hwopus: Leverage multistream API for decoding regular Opus packets

After doing a little more reading up on the Opus codec, it turns out
that the multistream API that is part of libopus can handle regular
packets. Regular packets are just a degenerate case of multistream Opus
packets, and all that's necessary is to pass the number of streams as 1
and  provide a basic channel mapping, then everything works fine for
that case.

This allows us to get rid of the need to use both APIs in the future
when implementing multistream variants in a follow-up PR, greatly
simplifying the code that needs to be written.