[SCSI] libsas: Provide a transport-level facility to request SAS addrs

Provide a facility to use the request_firmware() interface to get a SAS
address from userspace.  This can be used by SAS LLDDs that cannot
obtain the address from the host adapter.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>