scsi: libsas: Factor out sas_ex_add_dev()

Factor out sas_ex_add_dev() to be consistent with sas_ata_add_dev() and
unify the error handling.

Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>