Don't set the string "unknown" as a device's location_str

Return an empty string when the location is unknown instead of the
string "unknown". This ensures that all location entries are of
the form key=val.

Suggested by: imp
Approved by: jhb (mentor)
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21326