Fix SES on device slots without phys after r349321.

Broadcom 9400-8i8e HBAs report virtual SES device, where slots representing
external connectors are reported having no phys. Since sasdev_phys is NULL
there and proto_hdr is a union, ses_paths_iter() misinterpreted them as ATA.
Add explicit protocol check to properly differentiate them.

MFC after: 1 week
Sponsored by: iXsystems, Inc.