Fix "camcontrol rescan" with SATA drives behind a SAS controller

A bug in CAM's serial number hash logic resulted in SATA drives behind a SAS
controller getting removed and readded anytime the drive was rescanned for
any reason.

PR: 212914
Submitted by: kadesai
Reported by: kadesai
Reviewed by: asomers, ken
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp