sound: Fix minchn, maxchn and fmts in sndstat_get_caps()

The current implementation (incorrectly) passes the channel encoding
value to AFMT_CHANNEL(), which will always return 0, since the channel
number bits are masked out by AFMT_ENCODING().

Also add missing fmts initialization and aggregate encoding formats into
it directly.

Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45312