sound: Add OSS channel capabilities to sndstat nvlist

Assignment taken from dsp_oss_engineinfo().

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46166

(cherry picked from commit bd5bcc848c5764229926ad27a4bd77af4f87d189)