sound: Get rid of redundant variables in chn_setspeed() and chn_setformat()
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D48011
(cherry picked from commit 4e1b75bebf41b6e446d72c755fa420836341ade2)