mixer(3): Do not hardcode "/dev/mixer"

We have BASEPATH defined.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D43812

(cherry picked from commit 53c768e6836a32c8dcd0b0b422a169ef7a82a3ab)