ao_alsa: enable "plug" for non-interleaved float formats too

I have no idea what this code does, but it seems logical it should be
active for all float formats, not just for float with interleaved
access.
ao_alsa: remove unneeded checks

If initialization succeeds, p->alsa should always be set. Additional
checks are not needed, and also this wasn't even done consistently.
ao_oss: SNDCTL_DSP_CHANNELS takes int, not uint8_t

This caused weird issue, probably caused by setting up the wrong number
of channels, or similar. See github issue #383.

Patch by bugmen0t on github.
ao_oss: add 6.1 and 7.1 speaker placement from FreeBSD