sound: Return if the new speed/format is the same as the current one

This gives us a slight performance boost since we do not have to go
through chn_setparam() and the driver functions.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D48012

(cherry picked from commit 40616b7e41ff96b6d7522445b00f27f26d593a85)