mixer(8): Ignore mixer_open() failures for the -a option

The most likely reason mixer_open() will fail is because either the
device doesn't exist, or because it is disabled, so there is not reason
to kill the application. Instead, continue and print the rest of the
enabled mixers.

PR: 277615
Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45151