sound: Remove chn_timeout check from chn_init()

This check is not related to channel initializion, but is also
unnecessary, since sysctl_hw_snd_timeout() takes care of checking if
chn_timeout is within bounds.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44992