cam(4): Limit search for disks in SES enclosure by single bus

At least for SAS that we only support now disks are typically
connected to the same bus as the enclosure. Limiting the search
scope makes it much faster on systems with multiple buses and
thousands of disks.

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D32305