scsi: qla2xxx: Use predefined get_datalen_for_atio() inline function

 - Uses predefine inline function to access add_cdb_len field in ATIO.

 - Return SS_RESIDUAL_UNDER status when sending BUSY

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>