Accept any negative channel as autoselect.

And fix error codes in sceAudioChReserve().
Auto select the highest available channel.

This is what tests show, and what the PSP does.  It makes sense, since
the game might hardcode 0/1 but some library might use autoselect.  Or
something.
Return an error for invalid audio sample sizes.
Use a separate channel for SRC/Output2.

Tests on the PSP show that all 8 channels can be reserved/used at the
same time as *either* Output2 or SRC (but SRC and Output2 cannot be used
at the same time.)
Error when trying to reserve Output2 twice.
Validate the sample count in SRC and Output2.
Validate the SRC format, and enforce STEREO.

Were these actually outputting audio before?
Correct some logging to use ERROR_LOG.

Oops, was just copy and pasting the other wrong ones.
Merge pull request #1817 from unknownbrackets/audio-minor

Fixes to sceAudio Reserve funcs