sound: Merge pcm_chn_create() and chn_init()

Follow-up of b3ea087c05d8c75978a302cbb3fa92ce1afa3e49 ("sound: Merge
pcm_chn_destroy() and chn_kill()")

While here, add device_printf()'s to all failure points. Also fix an
existing bug where we'd unlock an already unlocked channel, in case we
went to "out" (now "out2") before locking the channel.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D44993