sound: Simplify chn_init()

[christos]

Summary

• Remove unnecessary CHN_[UN]LOCKs.
• Improve device_printf() messages.
• Remove redundant checks.
• Remove 0 assignments, since the channel is allocated with M_ZERO.
• Re-organize sections in a more coherent way.
• Remove "out1" label, just return NULL directly.
• Rename "out2" to "fail" and simplify logic.
• Do not check the return value of dsp_unit2name(), as it is guaranteed not to fail (we pass a valid channel type).

Sponsored by: The FreeBSD Foundation
MFC after: 2 days

[markj]

This is hard to review. I can't see any problems, but there are lots of things mixed in so it's hard to be confident. If you are happy with the change, I think you should just go ahead.

[dev_submerge.ch]

Don't see anything suspicious, no regressions in my (brief) tests.