sound: Call device_get_name() and device_get_unit() only once in mixer_init()

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D44840

(cherry picked from commit 41ab202ff74ea6af7e9125c9dfb430c729380f41)