mixer(8): Fix mixer status line for /dev/dspX.vpY mixer devices.

In some cases when passing /dev/dspX.vpY as mixer devices, m->ci.longname and
m->ci.hw_info will be empty. Don't print any brackets and parentheses
in this case.

Differential Revision: https://reviews.freebsd.org/D32500
Submitted by: christos@
Sponsored by: NVIDIA Networking