scsi: libsas: Factor out sas_ata_add_dev()

Factor out sas_ata_add_dev() and put it in sas_ata.c since it is a SATA
related interface. Also follow the standard coding style to define an
inline empty function when CONFIG_SCSI_SAS_ATA is not enabled.

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>