sound: Get rid of redundant assignments in chn_init()
c is allocated with M_ZERO.
Reported by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45273
(cherry picked from commit 2db2292dac7331be5414f86c3ef7863c7b6a4ac6)