iscsi-scstd: Annotate a fall-through for Coverity

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>

iscsi-scst/usr/iscsi_scstd.c

@@ -550,6 +550,7 @@ again:
 				conn->req.data = conn->buffer + conn->req.ahssize;
 				goto read_again;
 			}
+			/* fall-through */
 
 		case IOSTATE_READ_AHS_DATA:
 			conn_write_pdu(conn);