scsi: qla2xxx: use dma_mapping_error to check map errors

The return value of dma_map_single() should be checked by
dma_mapping_error(). However, in function qla26xx_dport_diagnostics(), its
return value is checked against NULL, which could result in failures.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/qla2xxx/qla_mbx.c

@@ -5809,7 +5809,7 @@ qla26xx_dport_diagnostics(scsi_qla_host_t *vha,
 
 	dd_dma = dma_map_single(&vha->hw->pdev->dev,
 	    dd_buf, size, DMA_FROM_DEVICE);
-	if (!dd_dma) {
+	if (dma_mapping_error(&vha->hw->pdev->dev, dd_dma)) {
 		ql_log(ql_log_warn, vha, 0x1194, "Failed to map dma buffer.\n");
 		return QLA_MEMORY_ALLOC_FAILED;
 	}